1. Technical Field of the Invention
The present invention relates to a document feeding apparatus, and more particularly, to a document feeding apparatus for feeding a bunch of stacked documents in one direction by picking them up one by one.
2. Description of Related Art
In recent years, it has widely been practiced to construct a copying system in an electrophotographic copying machine by combining an automatic document feeder for the purpose of improving efficiency in a copying operation. Accompanied with a requirement for speeding up a copy sheet feeding process in the main body of a copying machine, a high-speed document feeding technology is desired for an automatic document feeder as well.
In a document feeding technology, there are extremely delicate and difficult problems in providing a document feeding mechanism for surely separating and feeding a bunch of stacked documents in a fixed direction one by one without having oblique movement of the sheets, and there is a limit in speeding up a document feeding operation. In order to solve the problems, it is arranged to provide a feeding means possessed of sheet registering function, for instance, a register roller, at downstream side in a document feeding direction of separating means wherein a sheet of original separated into one sheet is brought in contact with a nip section of a pair of rollers of the register roller to form a loop at the leading end portion of the sheet, and after a posture of the original is adjusted, the original is fed at a high speed accompanied with a start of rotation of the register roller.
When a document feeding is started at a high speed by the register roller, however, if a separating means provided at upstream side in a document feeding direction is not being driven, the separating means acts largely on the document feeding operation as a large load until an original passes through the separating means to cause to damage the original, and generate a noise in a moment when a loop at the leading end portion of the original is dissolved. In order to solve such inconveniences, it is proposed that the separating means needs to be rotationally driven even when the original is fed at a high speed by the register roller whereby the damage of originals and the loop dissolving noise may be reduced. However, the separation capability of the separating means is lowered, and it may cause to induce an accompanied document feeding with other document.
More particularly, the separating means is arranged to obstruct the advance of second and third originals which tend to be fed together with a first original when the first original is fed by a transport velocity (first velocity) at the time of a separation process. However, when the first original is fed by the register roller at a second velocity which is higher than the first velocity, second and third originals are also run into the separating means with the second velocity together with the first original. The separation capability of the separating means does not effectively act on the following original being fed with the second velocity, and it is likely that the following original slips out of the separating means.
In order to cope with a high-speed document feeding operation, there is developed a preliminary document feeding technology wherein when an original picked up from a tray is fed, the following original is immediately picked up and fed to a predetermined position to stand by for the next operation. In this case, pickup means is set at an original pressing position when the original is picked up, and after the original is registered, the pickup means is retracted to a pressure releasing position. Further, when a preceding original is transported with a start of rotation of the register roller, the pickup means is moved to the document pressing position at a timing that the rear end of the original is detected by a sensor so that the above-mentioned troubles of original damage and the accompanied document feeding with other document can be prevented since the rear end portion of the preceding original is not remained on the tray. However, if a time is wasted for starting the pickup means until the rear end of the preceding original passes through a detecting point of the sensor, a document feeding process for the following original is delayed. It further takes time for original exchange to obstruct a high-seed copying operation in the main body of copying machine.
There is also developed a 2-in-1 mode as a high-speed processing technology when a plurality of originals are copied in the copying system. In the 2-in-1 mode, two sheets of originals are fed to an exposure position side by side on a platen glass, and images of two sheets of the originals are formed simultaneously on one copy sheet. In this 2-in-1 mode, the number of copy sheets can be decreased to half relative to the number of originals.
In the 2-in-1 mode, it is required to make a gap zero between two sheets of originals placed at an exposure position since stains of a transport belt is copied onto a copy sheet if there is a gap between the originals. Heretofore, it has been practiced to provide the register roller just in front of the platen glass, and under a state that the rear end of a preceding original is positioned at a nip section of the register roller, the original is temporarily stopped, and after the leading end of a succeeding original is brought in contact with the nip section of the register roller, both originals are simultaneously transported onto the platen glass side by side. In such apparatus, however, it is difficult to solve a problem of the appearance of a little gap between two sheets of originals by the width of the nip section of the register roller.
There is also introduced a type of apparatus wherein a preceding original is once transported onto a platen glass, and at the same time, a succeeding original is transported to the entrance to the platen glass. Then, the preceding original is backed to adjust the rear end of the original to the leading end of the succeeding original, and after they are transported simultaneously, both originals are placed side by side on the platen glass. However, it will take a longer time for an original exchange process since the original has to be switched back, and eventually copying efficiency is lowered.